1. Field of the Invention
The present invention relates to a pentagonal roof prism mounted in a single-lens reflex camera and, more particularly, to an improvement of a structure of an optical system provided for carrying out photometry before a photographing operation.
2. Description of the Related Art
Conventionally, there is known a pentagonal roof prism of a single-lens reflex camera, which serves as a part of a photometry optical system. Namely, a photoreceptor element for photometry is disposed behind a rear surface of the pentagonal roof prism, close to an eyepiece lens of the viewfinder optical system, and a condenser lens is interposed between the rear surface and the photoreceptor. Thus, a light beam passes through a focusing glass of the viewfinder, is reflected by an inner plane of the pentagonal roof prism, and emerges from the rear surface to become incident on the photoreceptor, so that a photometry value is sensed.
As described above, the pentagonal prism has a surface which should be a transparent surface so that a light beam is led to a photoreceptor, for example. However, depending upon the shape of the pentagonal prism, it is difficult to polish the surface to obtain a transparent surface, since a conventional tool for polishing a surface of a prism will barely fit on the surface.